/Engine/Plugins/Runtime/GeometryProcessing/Source/DynamicMesh/Public/NormalFlowRemesher.h
Test to see if flipping an edge would improve "normal error". Normal error for a triangle is: 0.5 * (1.0 - TriangleNormal  ProjectedNormal), where ProjectedNormal is the normal of the triangle returned by finding the closest point on ProjTarget. The normal error for an edge is the sum of errors for the edge's incident triangles.
True iff (CurrentEdgeError > BadEdgeErrorThreshold) and (NewEdgeError < ImprovementRatioThreshold * CurrentEdgeError)
bool EdgeFlipWouldReduceNormalError ( int EdgeID, double BadEdgeErrorThreshold, double ImprovementRatioThreshold ) const